COLUMBIA, MO - A seaplane broke apart and plunged into the waters off the coast of south Florida. An air tanker disintegrated and crashed while battling a California forest fire. A commercial airliner lost part of its upper fuselage before landing at a Hawaii airport. The official cause of each occurrence: material fatigue, which resulted in cracks that compromised the structural integrity of the aircraft.

At the University of Missouri-Columbia, a new and reliable method for identifying potentially dangerous cracks in aging aircraft has been developed. Using a vibration-based technique named the "Boundary Effect Evaluation Method (BEEM)" and a scanning laser vibrometer, P. Frank Pai, professor of mechanical and aerospace engineering in the MU College of Engineering, can detect tiny cracks within various materials - including aluminum alloy and composite laminates, which are the two primary materials used to build airplanes. He said the inspection can be done with the aircraft fully intact and for less money than the traditional method.

During laboratory tests, Pai attaches piezoceramic patches to composite laminate and aluminum alloy structures. With electrical voltage applied, the patches produce small vibrations. The laser vibrometer scans the structure at various uniformly distributed points, measuring vibration levels. The data is relayed to a computer and processed by a sophisticated mathematical theory developed by Pai, which is the basis of the BEEM. Once analyzed, the locations and sizes of the cracks are displayed on the computer's monitor.

Although other inspection methods exist, Pai's technique differs. First, the traditional ultrasound method requires the structural component to be disassembled from the aircraft, placed in water for an ultrasound scan and then reassembled. Pai's method allows for inspection of the entire aircraft, especially critical sections near the fuel tank and wing and body junctions, while fully assembled.

Secondly, he said many aircraft companies, as well as NASA, are developing structural damage inspection methods using high-power lasers to induce and measure stress waves. They produce high levels of heat, which Pai said can potentially damage the surfaces of structures. His method, which provides portability and can be transported to an airport or aircraft hanger for use, uses a mobile and low-power laser that causes no harm. He said it's less expensive and requires less time than the other methods.

Pai has tested his method on fuel tank structures of Navy aircraft, the upper wing panel from an F-16 fighter jet and numerous other structures, successfully locating cracks and defects unnoticeable to the human eye. He is attempting to present his method to the U.S. Air Force for future use. He also plans to make it available for structural integrity testing and the monitoring of aging mechanical systems and civil structures following natural disasters or wartime bombings.

Pai's study, "Dynamics- and Laser-Based Boundary Effect Evaluation Method for Damage Inspection of One- and Two-Dimensional Structures," has been published in the Journal of Nondestructive Evaluation.